The invention relates generally to cementitious compositions which are particularly useful as joint compounds, such as are used in finishing gypsum drywall panels. More specifically, the compositions are of the premixed settable-type, that is, they contain a cementitious settable base that has been premixed with water and prevented from setting, but will set and harden by chemical reaction with the water when a set initiator is added, although they may be used as drying-type joint compounds, if desired. The compositions also have other applications, including, but not limited to, firestop compounds, ornamental and casting plasters, spackling compounds, and base coat and finishing plasters.
Joint compounds are applied to complete the installation of gypsum panels by filling the space between adjacent panels, typically in combination with paper or fiberglass tape. The joint compound is forced into the space between the panels, either before or after the tape is applied. After the space between the panels has been filled, an additional coat or coats of joint compound, either the same as the first or formulated for the purpose, are generally applied to provide a smooth transition between the panels, after which the panels will usually be painted or receive some other surface finish.
Generally, joint compounds contain a filler, a binder, and a thickener. Conventional fillers include calcium carbonate, calcium sulfate dihydrate (gypsum), and calcium sulfate hemibydrate (plaster of Paris). Calcium sulfate hemihydrate sets by hydration to gypsum, while compounds using other fillers harden by simple drying. Thus, joint compounds are usually divided into two types, setting or drying, depending on whether the filler contains a substantial amount of calcium sulfate hemihydrate.
When a powdered setting-type joint compound is mixed with water, the calcium sulfate hemihydrate will react with the water to form gypsum. This occurs quite rapidly, usually between 5 and 300 minutes. Consequently, water cannot be added to powdered setting-type joint compounds until just prior to application to joints in the wallboard. To overcome problems of adding water at the time of use, joint compounds have been formulated to prevent the reaction of the hemihydrate with water so that premixed setting-type joint compounds can be manufactured. Setting compounds also have an advantage relative to drying-type joint compounds, since drying-type compounds will shrink during drying, requiring multiple coats and sufficient drying time between coats, making it difficult to achieve a smooth surface suitable for decorating in a timely manner.
Premixed setting-type joint compounds have been suggested in previous patents. For example, U.S. Pat. No. 4,661,161 discloses a set retarding composition in which a proteinaceous ingredient formed from animal tissues and a chelating agent for calcium ions is used. An accelerator including a compound having a higher log K value than calcium is mixed with the joint compound when setting is desired.
As noted in U.S. Pat. No. 5,746,822 ("'822 patent"), the shelf life of such ready-mixed setting joint compound is not sufficient since the activity of the set retarder was found to be degraded with time. Thus, the '822 patent discloses an improved setting-type joint compound which includes non-calcium bearing phosphate additives to prevent setting of a premixed joint compound which includes water. The shelf life is said to be at least several months. Again, compounds having cations having a higher log K value than calcium are employed as accelerators just before the joint compound is used to initiate the setting action. Each of the Examples and the allowed claims indicate that attapulgus clay should be included in the premixed joint compound.
U.S. Pat. No. 5,725,656 ("'656 patent") discloses an artificial marble plaster composition containing calcined gypsum which does not require the use of clays, but adds an acrylic latex and an aliphatic alcohol as set retarding agents. The objective appears to be to extend the setting time, rather than provide a premixed joint compound in which the calcium sulfate hemihydrate is prevented from setting, despite the presence of water. The maximum setting time reported for the preferred composition is 90 minutes. The '656 patent also indicates that addition of a melamine formaldehyde polymer assists in the retarding action.
U.S. Pat. No. 5,653,797 ("'797 patent") also discloses a formulation for a setting-type joint compound, which includes polymers having acrylic acid and acrylamide monomer units as set retarding agents. The polymers are said to be capable of providing a shelf life of about 2 years. Clays are recommended as "non-leveling agents" to improve the Theological properties of the joint compound and are included in each of the Examples.
U.S. Pat. No. 5,336,318 ("v'318 patent") discloses a drying-type joint compound which is clay-free. The problems associated with premature setting of calcium sulfate hemihydrate are not of concern in this formulation, however, since no hemihydrate is used. The disadvantages associated with clay are said to be avoided by replacing clay with in-situ associative thickeners, such as acidic acrylate copolymers. Advantages are stated to be improved viscosity stability, crack resistance, and shrinkage. The thickeners are used with a pH of 8 to 10, and addition of alkaline materials is suggested to provide the desired pH.
As discussed above, the premixed setting-type joint compound of the '822 patent includes clays as non-leveling agents and a substitute for the asbestos previously used. The present inventors have found that clays have a more important is role than originally recognized. Without clays, a premixed setting-type joint compound has a tendency to settle and harden. When settling occurs, some of the set prevention effect appears to be lost, and attempts to remix the joint compound have been unsuccessfiul. By contrast, settling of a drying-type joint compound can be reversed by mixing without affecting its performance.
The present inventors have found a further improved setting-type joint compound which, although it is clay-free, has superior properties and shelf life as discussed below.